I would say, in terms of balance, I would expect everything to be rebalanced from what it was. First off, obviously balancing for single player and multiplayer isn't exactly the same thing, and secondly, this is a whole new game, being made from the ground up. These concepts would be included at stage one of development instead of being added in later.
So when I say I want these things to return, it's not necessarily that I want them to be as over or underpowered as they were. I don't expect there to be any correlation between how powerful they were in multiplayer vs how powerful they'd hypothetically be in MEA single player. It's just that I liked it on a conceptual level, I had fun playing it, and I would like my single player protagonist to be able to play with some of these toys.
As for the Demolisher(the N7 Engineer), there honestly isn't a whole lot I want from that class. I feel it's primary gimmick, the supply pylon, was somethng that was very heavily geared towards the MP experience and the Horde Mode style in general. It is a turtle class. The elements I do like from it are it's omni-tool gimmicks, with the homing grenade it fires from the omni-tool, and the heavy melee being that it generates an omni-grenade and punches someone to detonate it in a shaped blast. I did like that it wasn't built around the drone, and I'd like to see more of that, but honestly I preferred the Talon Mercenary for engineers.
And I do understand that some of these classes - the Fury, the Talon, the Slayer & Shadow(perhaps ironically) are in that "Pink Mohawk", loud, stylistic, cool realm. But I don't feel that they're too over the top that they break immersion for me, and I enjoy playing as them. So even if they break immersion for other people, I'm going to play it selfish and argue for what I enjoy to make it into the game, because that line of 'immersion breaking' is arbitrary in that it's something that's just going to vary from person to person, and my willing suspension of disbelief is greater than others, I suppose.